Thermoplastic resins are a staple commercial product. Polyvinyl chloride (PVC), for example, finds use in such varied applications as flexible films and sheets, rigid foils and sheets, bottles and other containers, records, rigid extrusions such as pipes and conduits, cables, flooring materials, coatings for fabrics and for paper, and footwear. The PVC normally contains a filler in such applications.
One application of plasticized PVC is as an electrical insulation. Normally, a mixture of fillers will be used, for example a mixture of whiting, which is a low-cost filler, and a filler designed to impart the required electrical properties, such as calcined china clay (which is considerably more expensive than whiting).
Unplasticized PVC (UPVC) is now widely used to make rain-water products, such as pipes and guttering, as it is relatively cheap, light and corrosion resistant. UPVC is also used to manufacture pressure pipes, soil systems, drainage systems, ducts and conduits.
The UPVC will normally contain the following additives:
(a) a thermal stabilizer, usually a basic lead or tin salt;
(b) an internal lubricant (usually a wax of low melting point) to assist the PVC powder to absorb heat uniformly in the early zones of the extruder (i.e., to reduce melt viscosity);
(c) an external lubricant (usually a stearic acid ester) to assist the passing of melt through the die of the extruder;
(d) an impact modifier, especially for cold service conditions;
(e) a pigment (for example a mixture of carbon black and titanium oxide to produce a grey colour); and
(f) a filler (usually calcium carbonate, e.g. precipitated calcium carbonate (PCC)) principally to cheapen the compound. However, an important exception is in UPVC for use in pressure pipes; here, the use of a filler is usually avoided in order to prevent deterioration of the properties of the UPVC.
The nature of the filler (particle size and shape and its chemical constitution) is of importance in that it can affect such properties of the finished product as degree of gloss, air entrapment, permeability, fire resistance, chemical properties (e.g. resistance to acids and alkalis), ageing characteristics, mechanical properties (tensile strength, elongation, hardness, brittleness and the like), dimensional properties (e.g. creep and shrinkage) and processing characteristics.
The filler commonly used hitherto (calcium carbonate, which may be coated with stearic acid) is not without disadvantages. For example, it cannot, in general, be used in amounts greater than 10 phr (parts per hundred parts of resin), otherwise the acid resistance and brittleness of the finished product would be unacceptable.
In general, an increase in the amount of a conventional filler in PVC results in an increase in hardness, brittleness and level of water absorption and a decrease in chemical resistance. This degradation of properties can become apparent even at levels below 10 phr. The deterioration of properties can be so marked, as the level of filler is increased, that loadings as high as 15 phr would not generally be contemplated--except possibly with the addition of expensive adjuvants--in UPVC of good quality.
Furthermore, particulate calcium carbonate, as well as being difficult to handle in bulk can result in poor flowability in the filled resin which results in a tendency to clog the orifices of the extrusion apparatus. Thus, appreciable amounts of lubricant are required.
Moreover, the conditions for the processing of prior-art UPVC compositions are such that a heat stabilizer is required. The stabilizers used hitherto tend to be toxic; it would therefore be desirable to formulate UPVC compositions with a reduced amount of stabilizer.